In recent years, flat-sheet and hollow-fiber separation-membranes have come into use in the field of water treatment and the food field. For example, separation-membrane elements that include a separation membrane, and separation-membrane modules that include a plurality of such separation-membrane elements are used in water purification apparatuses. Separation membranes used in a separation process that uses separation-membrane elements are classified into microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, reverse osmosis membranes, forward osmosis membranes, and the like, in terms of their pore diameter and separation function. These membranes are used, for example, to obtain potable water from, for example, seawater, brine water, and water that contains a deleterious substance, to produce ultrapure water for industrial use, to treat wastewater, and to recover a valuable material. A membrane to be used is selected based on a substance to be separated and the ability of the membrane to separate substances.
Separation-membrane activated-sludge systems (membrane bioreactors: MBR) include separation membranes that are submerged in an activated sludge tank to separate the activated sludge and the treated water. MBR occupy a smaller space and achieve a good water quality. Thus, in Japan, MBR are being adopted mainly by small facilities, while, in other countries that have a number of new facilities, MBR are being adopted by large facilities that treat water of more than 100,000 m3 per day.
The activated-sludge process involves air introduced into the treatment tank to cultivate aerobic microorganisms in the tank. Fixation of an aeration block that includes an aerator under a membrane unit (hereinafter referred to as “element block”) allows a mixed stream of gas and liquid, the stream formed by aeration, to ascend through the element block to scour membrane surface fouling. Thus, the membrane unit can separate solid and liquid while cleaning membrane surfaces, which can provide for low-cost membrane-filtration. In this case, a combination of the element block and the aeration block is typically referred to as a separation-membrane module.
Conventionally, a flat-sheet separation-membrane element is formed by securing the periphery of a membrane (semipermeable membrane) by heat sealing or the like to both the front and the back sides of a strong, porous, flat support plate. And an element block is formed by arranging a plurality of such separation-membrane elements in parallel and inserting the arranged membrane-elements into a cuboid module-housing that has grooves therein and an opening at the top and the bottom.
Pocket separation-membrane elements without a support plate have been also proposed. Some of such elements are formed by sealing the perimeter of a separation membrane and creating a hole into which a permeate discharge pipe is fit. In a manner similar to the manner described above, a separation-membrane module is formed by arranging a plurality of such elements and communicating the elements with a discharge pipe (see, for example, Patent Documents 1 and 2).
Because the pocket separation-membrane elements without a support plate have a light weight and flexibility, the separation membranes flutter when the membranes are subject to a flow of water-to-be-treated (raw liquid), which discourages sludge attachment. However, when the separation membranes receive a large amount of water-to-be-treated, the membranes flutter intensely. Then, the membrane surfaces come into contact with each other especially at the upper end of the separation-membrane element, which causes serious problems that impair the separation function of the membranes, the problems including rupture and detachment of the separation membranes and the sealing portions.
To reduce flutter of a pocket separation-membrane element without a support plate, a method of securing the four corners of the element has been proposed. Such pocket separation-membrane has flexibility, but the four corners do not change their position. Thus, the membrane is blown up by the upward flow generated by aeration, and the upper portion vibrates more intensely, which causes the problem that adjacent membranes come into contact with each other and then the membranes are damaged (see Patent Document 3).
A method of applying tension to the four corners has been proposed to prevent slack. However, the constant application of the force stretches the membranes over time, and the tension force of the springs is reduced (see Patent Documents 3).